1. Field of the Invention
This invention relates to solid oxide fuel cell (SOFC) stacks and materials to address thermal gradients in all planes (x, y and z) of the stack. This invention further relates to contact materials which provide electrical conductivity between the electrodes and the separator plates disposed between adjacent fuel cell units. This invention further relates to cathode modifications whereby the operating temperature of the cathode of each fuel cell unit, as well as within a fuel cell unit, is matched with the temperature that it experiences based upon its location in the fuel cell stack and anticipated temperature gradient.
2. Description of Related Art
A solid oxide fuel cell is a solid electrochemical cell comprising a solid gas-impervious electrolyte sandwiched between a porous anode and porous cathode. Oxygen is transported through the cathode to the cathode/electrolyte interface where it is reduced to oxygen ions, which migrate through the electrolyte to the anode. At the anode, the ionic oxygen reacts with fuels such as hydrogen or methane and releases electrons. The electrons travel back to the cathode through an external circuit to generate electric power.
The constructions of conventional solid oxide fuel cell electrodes are well known. Electrodes are often composites of electron- and ion-conducting materials. For example, an anode may comprise electronic conducting nickel (Ni) and ionic conducting yttria stabilized zirconia (YSZ) and a cathode may comprise a perovskite such as La1-xSrxMnO3-δ (LSM) as the electron conducting material and YSZ as the ionic conducting material. Because of the high activation energy for oxygen reduction of perovskites, noble metals such as Au, Ag, Pt, Pd, Ir, Ru, and other metals or alloys of the Pt group may be added or used to replace the perovskite phase to reduce the activation energy as taught by U.S. Pat. No. 6,420,064 and U.S. Pat. No. 6,750,169, both to Ghosh et al. Furthermore, the noble metal may be alloyed to modify the optimum operating temperature range as disclosed by U.S. patent application Ser. No. 11/542,102 to Wood et al., issued as U.S. Pat. No. 8,313,875, which is incorporated in its entirety by reference herein.
Each individual fuel cell, also referred to herein as a fuel cell “unit”, generates a relatively small voltage. Thus, to achieve higher, more practically useful voltages, the individual fuel cell units are connected together in series to form a fuel cell stack. The fuel cell stack includes an electrical interconnect, or separator plate, typically constructed of ferritic stainless steel, disposed between the anode and cathode of adjacent fuel cell units, as well as ducts or manifolding, either internal or external, for conducting the fuel and oxidant into and out of the stack. In addition to separating adjacent fuel cell units, the separator plates distribute gases to the electrode surfaces and may act as current collectors. Electrically conductive contact pastes are used to bond the electrodes to the separator plates. U.S. Pat. No. 6,420,064 to Ghosh et al. discloses a cathode contact paste comprised of lanthanum cobaltate.
Conventional solid oxide fuel cells are operable at temperatures in the range of about 600° C. to about 1000° C., but generally exhibit high performance at operating temperatures only in the range of about 700° C. to about 1000° C. In a large scale, multi-layer solid oxide fuel cell stack, there can occur significant temperature variations in all planes, x, y and z. Stack temperature variations on the order of about 100° C. to about 200° C. have been measured. The ends of the fuel cell stacks are the coolest, resulting in low cell voltage and high degradation at the end cells when the cells in the center are operating in a reasonable temperature range that is not too hot for the ferritic stainless steel separator plates. On the other hand, when the stack is operated at sufficient temperatures such that the end cells are in a reasonable operating range, the center cells are too hot and excessive degradation occurs due to oxidation of the separator plates. For large area cells, it is easy to imagine similar effects in the x-y planes, leading to localized degradation or low performance.
Thus, there is a need for a solid oxide fuel cell stack having a materials system that can tolerate a large operating temperature range. In practice, this range may be too large for a single materials system and there may be a need for a simple, low cost method of modifying the materials system locally to accommodate temperature variations.